Electrophoretic casting of pottery

ABSTRACT

IN THE CASTING OF CERAMIC ARTICLES FROM SLIP IN POROUS MOULDS, CASTING UP IS AIDED BY ELECTROPHORESIS, THE ELECTROPHORETIC TREATMENT BEING COMMENCED AFTER A SHORT INITIAL CAST UP AND CEASING BEFORE A FINAL CAST UP WHICH HAS A MUCH LONGER DURATION THAN THE ELECTROPHORETIC TREATMENT. APPARATUS FOR CARRYING OUT THE PROCESS IS ALSO DESCRIBED.

Feb. 27, 1973 J. A. c. EBREY 3,713,554

ELECTROPHORETIC CASTING OF POTTERY Filed July 15, 1971 4 Sheets-Sheet 1 Feb. 27, 1973 J. A. c. EBREY 3,713,564

ELECTROPHORETIC CASTING OF POTTERY Filed July 15, 1971 4 Sheets-Sheet a Feb. 27, 1973 J. A. c. EBREY ELECTHOPHORETIC CASTING OF POTTERY 4 Sheets-Sheet 3 Filed July 15, 1971 WWW Feb. 27, 1973 J. A. c. EBREY 3,713,564

ELECTROPHORETIC CASTING OF POTTERY Filed July 15, 1971 4 Sheets-Sheet 4 WM WW W W United States Patent Office 3,718,564 Patented Feb. 27, 1973 U.S. Cl. 204-181 Claims ABSTRACT OF THE DISCLOSURE In the casting of ceramic articles from slip in porous moulds, casting up is aided by electrophoresis, the electrophoretic treatment being commenced after a short initial cast up and ceasing before a final cast up which has a much longer duration than the electrophoretic treatment.

Apparatus for carrying out the process is also described.

This invention relates to the casting of pottery,

Traditionally, pottery is cast by pouring an appropriate slip into a two or more-part plaster of paris mould, and allowing the slip to remain in the mould for a castingup period, during which the mould, which is porous, absorbs water from the slip adjacent its walls, causing a skin to form. The casting-up period is selected so as to provide a skin of appropriate thickness, and at the end of this period the remaining slip is poured off and the mould and the casting which remains in it are dried sufiiciently to enable the mould to be disassembled and the casting removed without damage.

This process is relatively lengthy, and although we have succeeded in developing machines in which the process is largely mechanized, these machines are large and bulky relative to their throughput because of the considerable length of the casting cycle, the ruling factor in which is the casting-up time. Particularly in the case of slips which do not have a china clay basis, this casting up time can be very long, which both entails, for a given output, a very large casting machine if the process is to be mechanized, and a very large number of moulds in use at any one time.

Proposals having been made to utilize the phenomenon of electrophoresis in order to accelerate the casting-up step. In electrophoresis, a finely divided material suspended in a liquid, e.g. a clay slip, is placed between two electrodes, the passage of a direct electric current between the electrodes resulting in a migration of the suspended particles towards one of the electrodes. In elect-rophoretic cast-ing, one of the electrodes, usually the positive electrode, is formed by the mould, the other electrode being dipped into the slip contained by the mould. It is found that by the use of electrophoresis a very large reduction in casting-up time can be achieved; for example, in a typical case using a metallic mould, a similar thickness of casting might be achieved after two minutes as was achieved in sixty minutes by traditional methods, and the advantage of this acceleration is obvious. However, various problems have prevented the adoption of this technique except possibly in certain specialized applications.

If traditional plaster of paris moulds are utilized, it is found that as a result of electrolysis, rapid deterioration of the mould takes place, and a mould which might be satisfactorily utilized forty or more times in a traditional casting process rapidly becomes useless, due to pitting and disintegration of its inner surface, after being utilized only two or three times for the electrophoretic process. This reduction in mould life renders the porcess totally uneconomic when tradition-a1 moulds are used. Moreover, we have also found that the inner surface of the casting produced, i.e. that remote from the wall of the mould, tends to exhibit unacceptable surface irregularities, rendering the castings produced much inferior in quality to those produced by the traditional method.

Attempts have been made to replace the traditional porous plaster moulds with non-porous metal moulds, but in this case not only may the problem of inner surface irregularities remain, but also there is a tendency for pitting to appear in the outer surface of the casting, caused it is believed by the evolution of oxygen at the positive electrode due to the electrolysis of water; the occurrence of such electrolysis is believed inevitable when water based slips are utilized.

The object of the persent invention is to enable the use of electrophoresis to reduce casting-up time during the casting of pottery in porous moulds, without unacceptable deterioration of quality of the castings or substantial reduction in mould life.

We have found that if slip is poured into a porous mould adapted to form one electrode of an electrophoretic cell, and a short casting-up period, hereafter referred to as the initial cast-up, is allowed before current is applied to the cell, if current at a predetermined potential is then applied for a short period, and a further casting-up period, hereafter referred to as the final cast up is allowed after cessation of the passage of current through the cell, then castings of good quality can .be obtained without substantial deterioration of the mould whilst still effecting very substantial reductions in the casting-up period.

According to the invention therefore a method of casting pottery hollow-ware comprises the steps of pouring slip into an assembled multipart mould made of porous material and having a substantial .wall thickness and an electrically conductive coating applied to its outer surface, allowing a short initial cast-up, passing a direct current between an electrode dipped into the slip and the conductive coating of the mould in such a sense as to cause electrophoretic deposition of solids from the slip upon the latter, the potential between the outer surface on the mould and the electrode being selected so as to be high enough to cause significant deposition and low enough to avoid the formation of substantial irregularities in the inner surface of the casting being formed, allowing a final cast-up for a period substantially greater than the duration of the passage of the electric current, draining surplus slip from the mould, partially drying the casting produced, and disassembling the mould and removing the casting.

The invention also extends to a casting machine comprising a conveyor supporting a plurality of trays for advancement through an endless series of stations in a stepwise manner, said stations comprising at least one filling station at which moulds carried by the trays are filled with slip, at least one initial cast-up station during residence in which initial cast-up is allowed to occur, an electrophoresis station at which a circuit is completed between electrodes dipped into the slip in each mould and the trays, which latter are electrically conductive and in contact with a conductive layer on the outer surface of the moulds, a plurality of final cast-up stations during residence in which final cast-up is permitted to occur, a plurality of tipping stations in which the trays are inverted so as to tip out surplus slip and allow draining of the moulds a plurality of drying stations in which the moisture content of the moulds and the casting they contain is reduced, one or more stations at which access is provided to the moulds to allow their disassembly or removal and reassembly or replacement, and at least one mould drying station during passage through which the moisture content of the moulds is reduced.

The invention is described further with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic plan view of a casting machine for moulding small pottery articles from porcelain, china, or earthenware slips,

FIG. 2 is an elevation of one of the trolleys shown in FIG. 1,

FIG. 3 is a part plan view of the same trolley, and

FIG. 4 is a section on the line A-A in FIG. 2.

Referring to FIG. 1, this shows diagrammatically a casting machine intended for the casting of small earthenware articles such as cream jugs, although it should be understood that the invention is by no means confined to the casting of small articles. It comprises a carousel type conveyor comprising a table 2, an endless chain 4 extending in a path about the central longitudinal axis of the table, and a number of trolleys 6 supported on the table by rollers 8 (see FIG. 2) and connected to the chain so as to be driven in a path around the periphery of the table. The chain is driven stepwise at preset intervals, for example 90 seconds, through steps equal to approximately half the pitch of the trolleys, with the result that with eleven trolleys as shown, a trolley takes 33 minutes to make a complete circuit of the table, passing through twenty-two different stations on the way. It should be clearly understood that the 90-second stepping interval described is by way of example only, and may be varied in accordance with the principles discussed later in the specification. Each trolley carries two trays each adapted to carry a row of moulds 7, so that on each step of the conveyor, all the rows of moulds are moved forward by one station.

In passing from one longitudinal run of the conveyor to the other, the trolleys pass through two successive stations A and B, in which stations an operative unloads moulds containing finished castings and reloads the trolley with empty moulds. With the next step of the conveyor, the first row of moulds on the trolley is advanced into a mould drier 10 consisting of a heated tunnel enclosing a portion of the conveyor table two stations (C and D) long. A row of moulds on leaving the drier 10, which adjusts the moisture content of the moulds to an appropriate level, passes in station E beneath a multiple filling head 12, which may be of an automatic type such as described in our copending patent application No. 17,172/70. Passage of the row of moulds through the following two stations F and G allows an initial cast-up of three minutes.

At the following station H, a gantry 14 carrying a row of electrodes 16 is lowered so that the electrodes enter the slip contained in the moulds and electrophoresis occurs. Further description of this step is given below. The electrophoretic treatment lasts for a minute and a half, and the row of moulds subsequently passes through stations I, J, K, L, M and N, ta king a further nine minutes and allowing a final castup to take place.

In the following station the row of moulds is inverted, so as to tip surplus slip into a tank 18 situated below the table through a tunnelling arrangement 20 (see FIG. 2). In the following stations, P, Q, R, S and T the row of moulds remains inverted, returning to its normal orientation and it enters station U. Stations U and V are within a further mould drier 22, which is adapted to cause sufficient drying of the moulds to enable the castings to be removed from the moulds without damage. The row of moulds then re-enters station A and the entire sequence is repeated.

Since the carousel conveyor is of conventional construction it has not been described in detail. However, the trolleys carrying the moulds are of special construction and are further described with reference to FIGS. 2-4. Each trolley 6 comprises a generally rectangular frame 24 supported by rollers 8 and in turn supporting two pairs of bearing blocks 26 in which are journalled stub axles 28, 30 supporting trays 32, two for each trolley, by means of end frames 34. In the example shown, each tray 32 supports a row of three moulds, the bases of the moulds resting on copper strips 36 secured to the tray 32. The moulds are clamped in place on the tray 32 by means of a series of clamping bars 38 extending between end plates 40. The end plate 40 at one end of the clamping bars is hinged to a bracket 42 adjustably clamped to a pillar 44 carried by one end frame 34. The other end plate 40 carries a clamp 46 equipped with a locking wheel 48 and adapted to engage a pillar 50 carried by the other end frame 34. In order to permit loading and unloading of the moulds, the handwheel 48 may be unscrewed and the assembly of the end plates 40 and the clamping bars 38 swung back about the hinged connection to the bracekt 42, Whilst when the clamp 46 is engaged with the pillar 50 and locked by the wheel 48, the moulds are clamped firmly in place against the strips 36. The moulds themselves are conventional plaster of paris moulds except that their external surfaces are sprayed with a coating of a low melting point alloy, the coating being suificiently thick to have good electrical continuity, without being so thick as to prevent moisture from escaping from the mould.

Various substitutes for plaster of Paris for the manufacture of porous moulds have been proposed, and those substitutes which give satisfactory results in the conventional casting process are likely to be satisfactory in the process of the invention, provided that they are not more prone than plaster of Paris to electrolytic damage. The nature of the alloy used is not critical, but alloys should be avoided which are found to electrolyse and contaminate the mould with traces of metallic salts which may manage to percolate through the mould and reach the slip being cast and which on firing would cause discolouration. We have obtained good results with an alloy of tin, bismuth and zinc. It is also desirable that the mould should be designed so as to have a substantial and preferably, though not necessarily, fairly even wall thickness. A wall thickness greater than 1% inches is preferred for optimum mould life, and with this as a minimum thickness, it is no longer essential that the wall thickness be uniform though in fact this is normal practice. Each of the stub axles 28 carry earns 52 adapted to engage a cam track running parallel and above the chain 4 through the stations 0, P, Q, R, S and T (see FIG. 1) so as to tip the associated tray 32 to a substantially inverted position whilst passing through the stations. The trolleys 6 are connected by brackets 54 to the chain 4 which is guided for movement by a box member 56. If the chain 4 is of the biplanar type with sets of supporting rollers on two perpendicular axes, the inboard rollers 8 may be dispensed with.

As will be better appreciated after consideration of the discussion below, the number of stations provided by the conveyor, and their distribution between the different stages of the process may be varied according to requirements; factors which may influence this distribution are in particular the size of the articles being cast, since normally the time required for each stage of the operation will increase as the size of the articles being cast increases, and also the nature of the slip being used, since the casting-up times required can vary widely with different types of slip. In particular, porcelain slips require rather shorter casting-up times than earthenware slips, and slips based on china clay require shorter casting-up times than slips based on other commonly used clays.

It will be appreciated that the various stages of the casting process apart from the casting-up stages are basically conventional, but further consideration will be given to the casting-up stages.

The initial cast-up period need only be long enough to form an initial skin on the interior surface of the mould, and a period of a few minutes is sufiicient. This period is not particularly critical, and if a casting machine such as that described is used, it may be geared to the stepping period of the conveyor so as to last some 2-4 minutes. The critical step is the electrophoretic treatment. In the machine described, the metallic coating on the outer surface of the moulds provides one electrode for this treatment; this will be the positive electrode with all conventional slips. The negative electrodes are supported by a gantry which is raised and lowered in synchronism with the stepping mechanism of the conveyor, and is preferably provided with an electrical interlock so that the electrodes are not energised except when the gantry is in its lowered position. The gantry should of course be associated with appropriate guard enclosures and be properly insulated so as to ensure electrical safety, particularly in view of the fairly high" potentials utilized as described below. A suitable variable potential direct current electrical supply is provided, the positive line of which is earthed and connected to the'conveyor structure for transmission therethrough to the moulds via the copper strips 36, and the negative line is connected to the electrodes 16. The construction of the electrodes themselves is not critical, though the gantry should be adjusted so that when the electrodes are lowered, they do not come too close to the walls or bottomv of the mould and are as far as possible symmetrically placed therein. We have found that satisfactory results are obtained when the electrodes are formed of lengths of half inch copper tube; no problems have been experienced with discolouration of the slip.

The current passed during the electrophoretic treatment is conveniently controlled by adjusting the potential applied between the electrodes, and we have found that using conventional china clay based earthenware slips, a potential in the range 200-300 volts is satisfactory for smaller articles, with a slightly higher range being suitable for larger articles. The preferred range of voltage for smaller articles, such as cream jugs of conventional design, is 230-250 volts, rising to 250-4280 volts for larger articles such as coffee jugs. Too low an applied voltage results in the electrophoretic treatment being comparatively ineffective, resulting in only a small saving in casting-up time, whilst too high an applied voltage results in the formation of characteristic wave patterns on the inside surface of the casting. It should be understood that the above figures are given for guidance only and experience with differing types of slips, or slips containing different contents of dispersing agents or electrolytes, or different sizes and shapes of articles, may dictate some variation. Surprisingly, we have noted that the results obtained appear to be much more dependent upon the voltage applied than the current passing through the mould; this latter, we have found in the course of our own experiments, may vary according to circumstances between .2 and 1.5 amps per mould.

Both the potential applied and the duration of the treatment should be as small as possible consistent with effective results, since unduly high potentials (and therefore increased currents) and unduly long durations of treatment cause a substantial reduction in mould life. We have found that, using plaster of Paris moulds, and applying current for one and a half minutes at 230 volts, a conventional mould (provided with the metal spraying described above) for a cream jug is still usable after some forty moulding cycles, which is entirely acceptable.

After the electrophoretic treatment has been discontinued, and during the final cast-up, we have found that the rate of casting during this period is nevertheless increased subsequent to the electrophoretic treatment. Whilst we are uncertain of the exact reason for this phe nomenon, it is extremely valuable in reducing the length of the electrophoretic treatment required whilst at the same time enabling a substantial reduction in the total casting-up time required. In the apparatus specifically considered above with reference to the drawing, which is suitable for small articles, the total casting-up time including the electrophoretic treatment is reduced to 12 /2 minutes as opposed to some 30 minutes to one hour in conventional processes for similar articles using the same slip. Proportionate savings in casting-up time can be achieved in the casting of larger articles. This reduction in casting-up time is particularly beneficial when slips are used which are slow casting, for example those earthenware slips widely used in Europe which are based on clays other than china clay and whose casting-up time is v ry substantially greater than that of china clay slips. This very long casting-up time has proved a severe obstacle to the mechanisation of the casting process when such slips are used, due to the very large size of casting machine that would be required using the conventional casting process if a reasonable output is to be achieved. Obviously, if the length of the casting-up period can be reduced, the length of the casting cycle and thus the size of machine for a given throughput is reduced.

A further incidental benefit of the electrophoretic treatment is that the dissipation of electrical energy that occurs in the moulds and their contents causes an appreciable rise in their temperature, which tends to decrease the time required for subsequent processing stages. Whilst a machine for casting small articles and based on a carousel conveyor has been described, larger articles can be cast using a suitably enlarged machine of the same general type. Moreover, the machine need not be based on a carousel conveyor but may instead be based on other forms of moving moulds by steps through a plurality of processing stations. For example, if the invention is applied to the casting of sanitary earthenware, a conveyor system such as that described in our copending application No. 24,67 8/ 70 may be employed.

What is claimed is:

1. A process for casting pottery hollowware utilising electrophoresis comprising the steps of pouring slip into an assembled multipart mould made of porous material and having a substantial wall thickness and an electrically conductive coating supplied to its outer surface, allowing a short initial cast-up, passing a direct current between an electrode dipped into the slip and the conductive coating of the mould in such a sense as to cause electrophoretic deposition of solids from the slip upon the latter, the potential between the outer surface on the mould and the electrode being selected so as to be high enough to cause significant deposition and low enough to avoid the formation of substantial irregularities in the inner surface of the casting being formed, allowing a final cast-up for a period substantially greater than the duration of the passage of the electric current, draining surplus slip from the mould, partially drying the casting produced, and disassembling the mould and removing the casting.

2. A process according to claim 1, wherein the initial cast up time is 2-4 minutes.

3. A process according to claim 1, wherein current is passed during the electrophoresis step by applying a potential of 200-300 volts between the electrode and the mould coating.

4. A process according to claim 3, wherein the potential applied is about 230-250 volts to about 250-280 volts, rising with the size of the article being moulded.

5. A process according to claim 1, wherein the duration of the passage of current is as short as possible consistent with effective results.

6. A process according to claim 3, wherein the duration of the electrophoretic treatment is about 1% minutes.

7. A process for casting pottery hollowware comprismg advancing a plurality of trays for advancement through an endless series of processing stations in a stepwise manner, said stations comprising at least one filling station at which porous moulds carried by the trays are filled with slip, at least one initial cast-up station during residence in which initial cast-up is allowed to occur, a plurality of further cast-up stations during residence in which further cast-up is permitted to occur, a plurality of tipping stations in which the trays are inverted so as to tip out surplus slip and allow draining of the moulds, a plurality of drying stations in which the moisture content of the moulds and the casting they contain is reduced, one or more stations at which the moulds are selectively disassembled or removed and reassembled or replaced, and at least one mould drying station during passage through which the moisture content of the moulds is reduced.

the improvement wherein an electrophoresis station is provided between the initial and the further cast up stations, at which electrophoresis station electrodes, are introduced into the slip contained by said filled moulds, and connected to a source of electrical potential between said electrodes and said trays, the latter being electrically conductive and in electrical contact with conductive coatings applied to the exterior of said moulds. 8. A process according to claim 7, wherein at the electrophoresis station the electrodes are lowered into the slip from a gantry carrying an electrode for each mould oc- 8 cupying the station at any one time, the electrodes being lowered into the moulds and retracted therefrom in synchronism with the stepping of said trays.

9. Process according to calim 7, wherein the conductive layer on the moulds is produced by spraying of a low melting point alloy.

10. Process according to claim 7, wherein the trays are provided with electrically conductive contact strips and the moulds are secured in position on the contact strips.

References Cited UNITED STATES PATENTS 3,476,666 11/1969 Bell et a1 204'181 FOREIGN PATENTS 505,709 5/1939 Great Britain 204-18l 1,115,222 5/1968 Great Britain 204-181 HOWARD S. WILLIAMS, Primary Examiner 

